Open Image Annotation Initiative
The Open Image Annotation Initiative is planned to be a set of tools to allow you to annotate any image with sketches and embed those drawings into the file itself, while keeping the original image intact.
It does not use any new or proprietary format.
Any image file type.
With pen tablet support.
Read more here.
See how other people implement this idea.
Getting the Python version of the tool up and running is the first priority. Here are the plans for that.